Aircraft typically travel to altitudes where the ambient environment contains insufficient oxygen for normal respiration. For this reason, they are usually constructed to be airtight during operation. Thus, when in flight at high altitudes, they can be internally pressurized, resulting in a more comfortable environment for the operators and passengers. Aircraft are typically constructed to withstand such pressurization, where the internal pressure exceeds the external pressure.
Internal pressurization is typically controlled and managed by the operator of the aircraft. During flight, the air pressure within the pressurized aircraft is greater than the external pressure at normal travelling altitudes. Thus, concern over maintaining integrity of the aircraft typically centers on retaining air within the aircraft.
Under certain circumstances, however, the air pressure within the aircraft can be less than that of the surrounding environment. As one example, the internal space of an aircraft can have a higher air pressure than the ambient pressure during flight, but that higher pressure environment can be less than the ambient pressure at a planned landing site.
Increased external pressure is typically mitigated through the use of a negative pressure relief valve. Such a relief valve can be constructed to permit fluid, such as air, from the external environment to enter the aircraft when the pressure differential exceeds a certain amount. Such valves, however, can be exposed to harsh conditions, including the ambient environment, weather, stress fatigue, and so on. When a negative pressure relief valve is compromised, it can substantially affect the normal operating performance, which in turn affects the pressure differential experienced by the aircraft. Therefore, it is desirable to have a negative pressure relief valve that maintains significant operating capability regardless of its physical and structural integrity.